What has been proposed is a fuel cell that generates electromotive force by comprising a hydrogen pole and oxygen pole that sandwich an electrolytic layer that transmits hydrogen ions and which causes a reaction expressed by the following reaction formulas with the hydrogen pole and oxygen pole, respectively.Hydrogen pole: H2→2H++2e−Oxygen pole: (1/2) O2+2H++2e−→H2O
For a system that uses this kind of fuel cell as a power supply, hydrogen gas must be supplied on the hydrogen pole side. For supplying hydrogen gas, there is a method that directly uses reserve hydrogen gas using a hydrogen occluded alloy or the like, and a method that uses hydrogen that is drawn by a chemical reaction such as a reforming reaction from a specified raw material such as methanol and natural gas prepared as fuel. Fuel such as natural gas is generally decomposed to a mixed gas that contains hydrogen by a reaction such as the one shown by the following formulas.CnHm+nH2O→nCO+(n+m/2) H2;CnHm+2nH2O nCO2+(2n+m/2) H2;
It is also possible to directly supply mixed gas to a fuel cell, but in addition to reactions at the electrode being hindered due to a drop in the hydrogen partial pressure at the electrode, there is also the risk of other ill effects such as having stable reactions being hindered due to toxification by carbon monoxide at the electrode, so normally, the mixed gas is supplied to the fuel cell after a process for reducing the carbon monoxide concentration or a process for separating only hydrogen is performed.
For these reactions, to separate hydrogen from the generated gas, a hydrogen separation film that has a property of selectively transmitting only hydrogen is used. Items known as hydrogen separation films are films formed with palladium or a palladium silver alloy, or items for which these metals are coated on a porous support medium such as ceramic. A hydrogen separation film has a property of moving hydrogen in a film by the difference in the hydrogen partial pressure on the front side and back side of the film, and can separate hydrogen from mixed gases using this property.
Specifically, as technology relating to a hydrogen separation film that uses a porous support medium and the manufacturing method thereof, there are the technologies noted in JAPANESE PATENT LAYING-OPEN GAZETTE No. 1-266833 and JAPANESE PATENT LAYING-OPEN GAZETTE No. 63-171617, for example. The former is technology that relates to the method of manufacturing a hydrogen separation film that suppresses the generation of pin holes, whereby a thin film of metal LaNi5 that has hydrogen separating properties is formed using a sputtering method on a permeable porous substrate formed by a sintered body of stainless steel metal powder, etc. With this technology, when doing this, by adjusting the sputtering emission direction, a hydrogen separation film with no pin holes is formed on the surface layer of the porous substrate.
The latter is technology that tries to improve things such as transmissivity, heat resistance, and separation properties at high temperatures by forming a hydrogen separation film on an inorganic porous film with palladium supported. By loading palladium using a pressure reduction degassing method onto an inorganic porous film that underwent vapor deposition processing with palladium in advance, palladium is supported in the pores, and a hydrogen separation film is formed. More specifically, proposed are items such as a thin film with a palladium thin film formed on the surface of a porous film with part of this going into the pores as an anchor, and a thin film for which fine particle palladium is supported in the inner walls of the pores.
However, with hydrogen separation films of the prior art, there were problems in that the film thickness was thick and the hydrogen transmission speed was slow. There were also problems of it being easy for pin holes to occur in the hydrogen separation film and mixed gas leaking on the purge side.
For example, with the technology noted in JAPANESE PATENT LAYING-OPEN GAZETTE 1-266833 described above, a hydrogen separation film is formed on the surface of a porous substrate, so the overall thickness of the hydrogen separation part unavoidably became larger. Also, if an attempt was made to sufficiently prevent pin holes, the film thickness of the hydrogen separation film again became unavoidably thicker. The hydrogen separation film noted in JAPANESE PATENT LAYING-OPEN GAZETTE 63-171617 has palladium supported into the pores of the porous film, but with a palladium thin film formed on the surface, part of this entered the pores, or fine particle palladium is supported in the inner walls of the pores, and it does not extend beyond this level, so the effect was not sufficient.
However, in recent years, studies have been done for incorporating fuel cells in things such as vehicles, so the demand for fuel cell systems to become more compact is especially marked. It is known that the transmission speed for the hydrogen separation film is proportional to the surface area of the separation film and inversely proportional to the film thickness, and when using a hydrogen separation film of the prior art, in addition to devices becoming larger due to the film thickness being thick, it becomes necessary to compensate for the thickness of the film using the surface area, so it was not possible to make the separation part sufficiently compact. Also, to avoid gas leaks due to pin holes, the film thickness tends to become thicker, which is an obstacle to making the separation part more compact. Moreover, when forming a fuel gas generating device without using a hydrogen separation film, it is necessary to have something like a part that reduces the concentration of carbon monoxide contained in the mixed gas, which causes the device to become even larger. To try to make this kind of fuel cell system more compact, it is necessary to use a hydrogen separation film in the fuel gas generating device, and to satisfy the requirement of making devices markedly more compact in recent years, it has become necessary to further increase the hydrogen separating performance and to reliably avoid ill effects due to pin holes.